This invention relates to an improved method of installing an elongated heater such as a cartridge heater in a molding member such as an injection molding manifold member.
As is well known in the injection molding art, uniform and accurate provision and control of melt temperature is becoming more and more important as improved systems and techniques permit and, in fact, demand the molding of increasingly difficult materials. It is, of course, well known to heat manifold members using elongated cartridge heaters of the type disclosed in U.S. Pat. No. 2,831,951 which issued Apr. 22, 1958 to Watlow Electric Manufacturing Company. These cartridge heaters are inserted into bores in the manifold members to extend along the hot runner passage extending through the manifold member to maintain the melt at a constant uniform temperature. However, these systems suffer from the disadvantage that there necessarily are small air spaces and/or layers of oxide left between the outside of the cartridge heaters and the bores in the manifold member into which they are inserted which has an insulative effect and substantially reduces the effectiveness of the heaters. Furthermore, the dimension of the air space or oxide layer is not uniform along each heater with the result that heat is not applied evenly along the hot runner passage, nor is it transferred evenly away from the electric heating element in the heater. This results in the formation of hot spots along the heating element which all too frequently causes it to burn out, necessitating a costly shut down of the system to replace it.
The properties of some of the new engineering materials with operating melt temperatures in the 900.degree. F. range require that manifold members have increased loading capacity ratings. A recent attempt to overcome the problem has been to insert a tapered sleeve around the cartridge heater in a tapered hole. While this has reduced the size of the insulative air space, it has not eliminated it and, in fact, results in there being two air spaces, albeit they are smaller in size. In an alternative solution to this problem, the applicant provides a system with a tubular heating element cast into channels in the surface of the manifold member as disclosed in Canadian patent application Ser. No. 393,671 filed Jan. 6, 1982. Similar advantages arise in providing improved heat transfer from the tubular heaters arranged in a heater plate of a thermosetting mold.